1. Field of the Invention
The present invention generally relates to optical illumination systems in which a condensing lens is used, and more particularly to an illumination system for an overhead projector, which is constructed to maximize the amount of illumination over any desired region or aperture for a given optical geometry.
2. Description of the Prior Art
Condenser lenses for optical illumination systems are commonly used to collect light from a source and direct it toward a region or aperture to be illuminated. Such lenses may include single or multiple elements designed in bi-convex, plano-convex and meniscus shapes. One common use of condensing lens systems is in overhead projectors (OHP's).
A conventional OHP generally comprises a base having a light source, a stage area and a projection head located above the stage. The light source often consists of either a reflectorized lamp or a lamp which has no reflector in combination with a separate reflector and possibly a condenser lens. The Fresnel lens at the stage gathers the light and directs it to the projection lens in the head, which projects any transparent image placed on the stage.
It is desirable to provide as bright an image as possible in order to ensure that it is adequately visible in light environments. Brightness has become particularly important with the increasing use of liquid crystal display (LCD) panels in conjunction with OHP's, since the transmission of an LCD panel is typically very low.
Uniformity of illumination is also important. Although the human eye is not very sensitive to gradual changes in illuminance, changes of more than a factor of about four from the center of the screen to the remote corners are discernable. Uniformity is usually measured according to ANSI/ISO 9767-1990, which defines it in terms of a corner-to-center ratio. A corner-to-center ratio of greater than 40% is considered very good for an OHP according to the Japanese standard, JIS B 7160, which is essentially identical to the ANSI definition of corner-to-center ratio.
Three important factors involved in achieving high stage illumination are: lamp construction, collection of the light from the lamp, and utilization of the light once it has been collected. The first factor involves the lamp itself while the other two factors are dependent on the performance of the condenser lens or lenses. Lamp construction can be varied in several ways to yield increased illumination. Conventional OHP lamps are either incandescent or arc discharge. An incandescent lamp is generally lower in cost than an arc discharge lamp, and therefore is more widely used in overhead projectors, although arc lamps can provide higher illumination. If an incandescent lamp is chosen, the only available means by which the illumination can be increased is to use one with a higher wattage. Wattage is still limited since other components of the projector may become overheated. For example, if the Fresnel lens/stage becomes too hot, it can distort the image bearing film.
With respect to the other illumination factors, the collection and utilization of the light may best be achieved with an appropriate condensing lens system. A condenser lens can be constructed to collect as much of the light from the lamp as possible and direct it through the stage. The amount of light collected by the condenser lens is governed by its collection angle, which is defined as the angle between two rays, one passing through the upper edge of the collection system and the other passing through the lower edge. This angle is affected by the distance from the filament of the light source to the closest side of the condenser lens. The condenser lens is generally placed as close to the lamp as is practicable, so the only other factor involved is the diameter of the condenser lens. Increasing the diameter automatically increases the collection angle, but this too has a limit since it also tends to increase the thickness of the condenser lens in the process, thereby increasing its cost and susceptibility to thermal stress. Therefore, a balance between the diameter of the lens with its thickness to give high collection efficiency is needed. There are a number of patents in the art which discuss the amount of light collected by a condenser system.
The main function of a condenser is to collect light from a source and aim it toward the region or aperture to be illuminated. In the prior art, the amount of light collected by the condenser system is described by its relative aperture (U.S. Pat. No. 934,579), numerical aperture (cf. U.S. Pat. No. 2,637,242) or collection angle (cf. U.S. Pat. Nos. 1,615,674, 1,946,088 and 2,587,956). The collection angle is defined as the angle between two rays, one passing through the upper edge of the collection system and the other passing through the lower edge.
One common feature in condenser designs is that the system is aplanatic, that is, corrected for spherical aberration and coma (cf. U.S. Pat. No. 934,579 and 1,507,212). Aplanatism is especially desirable in illumination systems for high resolution optical systems such as microscope illuminators. The need for correction of coma is decreased for lower resolution systems and may be intentionally avoided as in U.S. Pat. No. 2,637,242. Another common feature is that all condenser systems attempt to achieve some measure of uniformity of illumination (cf. U.S. Pat. Nos. 2,637,242 and 5,010,465). Ideally, the designer should be able to specify the uniformity produced by the condenser. It would, therefore, be desirable and advantageous to devise a single element condenser lens which increases illuminance in illumination systems without increasing the size and thickness of the condenser lens, and yet still maintains high uniformity of the projected light.